Method for the determination of the content of isobutane in a mixture comprising essentially isobutane and nu-butane



Nov. 24, 1942. A w R 2,303,265

METHOD FOR THE DETERMINATION (5F THE CONTENT OF ISOBUTA NE IN MIXTURES COMPRISING ESSENTIALLY ISOBUTANE AND N-BUTANE Filed June 28, 1941 20 a0 4 60' m 60 90- I00 INVENTOR Patented Nov. 24, 1942 METHOD FOR THE DETERMINATION OF THE CONTENT OF ISOBUTANE IN'A MIX- TUBE COMPRISING ESSENTIALLY ISO- BUTANE AND N-BUTAN E Alfred W. Francis, Woodbury, N. J., assignor to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application June 28, 1941, Serial No. 400,267

2 Claims.

This invention has to do with a method of analyses and control capable of rapid determination of the amount of is'o-butane in a mixture of iso-butane and n-butane and so applicable to the control of processes for theisomerization of butane.

Several processes are in existence or are being developed for isomerization of normal or straight chain parafiin hydrocarbons such as butane to produce corresponding branched chain or isocompounds such as iso-butane, useful in high compression motor fuels. Many of these processes are capable of such operations as to produce little or no cracking to lower or higher hydrocarbons. Conversion to iso-butane is never complete, since the equilibrium mixture contains about 70 to 80% of iso-butane and about 20-30% of n-butane depending upon the temperature; and commercial operation is usually stopped considerably short of equilibrium to conserve time. It is therefore necessary to resort to some method for the analysis of mixtures of iso-butane and n-butane in order to properly control such processes.

It is customary to analyze gaseous mixtures of this sort by low temperature fractional distillation, as in the Podbielniak method. Such a method requires expensive apparatus, highly trained technique, about three hours of time, and about grams of sample, rendering it rather inadequate as a method-of. operating control.

Critical solution temperature between aniline and-hydrocarbons is used in some cases to characterize hydrocarbons,but;in the case of the butane the temperatures are so high (84.1 C. for butane, 109 C. for iso-butane) that the method would need be conducted under excessive pressure and would not be applicable.

This invention has for its object the pr. vision of a method for determining the proportion of iso-butane in a mixt e consisting essentially of iso-butane and norm 1 butane which method is rapid and of high accuracy.

This invention is based upon the discovery that critical solution temperatures of certain solvents, for example, ortho-nitrotoluene, with the butanes can be used to give quick and accurate values for' ting rapid and accurate observations, and the solvents are cheap and obtained readily in a nearly pure state.

The apparatus consists of glass tubes or ampules, suitably 10 to 20 cm. long and 5 to 8 mm. in diameter, sealed at one end and drawn down to a narrow neck at the other, about 1-2 5 mm. diameter. These are filled about full with the solvent, for example, pure orthonitrotoluene, through a capillary funnel, and immersed in a cold bath of dry ice and acetone (which freezes the ortho-nitrotoluene). A sample of butanes to be analyzed is condensed into a tube through a .capillary funnel, filling the tube to about 1.5 times the depth of the solid ortho-nitrotoluene, i. e., the tube is about 75% filled.

The tube is then sealed at the neck with a fine flame of a blast lamp while the tube is still immersed in the cold bath. The tube is then withdrawn, attached by rubber bands to a thermom eter reading preferably infractions of -a degree,

and immersed in a water bath, suitably a large glass test tube; By frequent tipping of the whole tube with gradually rising or falling temperature,

the point of disappearance or reappearance of cloudiness can be observed visually within one or two tenths of a degree centigrade. The test requires ten to fifteen minutes. The composition of the butanes is a linear function of the critical solution temperature. Typical results 30 with ortho-nitrotoluene are as follows:

Example Iso-butane Critical solution temperature Made up Found Per cent Per cent 12.5 C 0 l 0 13.3.- 4 2 4. 0 20.5. 39. 8 39. 4 40 27.0.. .4 71.4

1 Assumed. 1 Podbielniak. I The attached drawing shows in graphical form the. percentage of iso-butane plotted against critical solution temperature, using ortho-nitrotoluene as a solvent The solvent to be used is one having a critical solution temperature with pure iso-butane which is less than about 70 C. and also having a spread between that critical solution temperature and the critical solution temperature with pure normal butane of at least about 10 C.

Several suchsolvents and the respective critical solution temperatures for the two butanes are shown below:

More accurate analyses of course result with materials having the greater spread. Acetophenone is least desirable, since, due to its freezing point being 19.'l 0., its use necessitates super-cooling.

The method is obviously of a rapidity and accuracy capable of utilization for the control of operating processes, even in the hands of relatively unskilled technicians.

I claim:

1. That'method for the determination of the content of isobutane in a mixture comprising essentially iso-butane and n-butane which comprises the steps of liquefying the mixture, placing a portion of the liquid mixture in contact with a somewhat less volume of a liquid solvent substance-in a sealed ampoule, the said liquid solvent substance having a critical solution temperature with pure iso-butane of not more than about 70 C. and having a critical solution temperature with pure n-butane not less than 10 C. difierent from that with iso-butane, cooling the ampoule slowly through the temperature range appropriate to the solvent, and observing the critical solution temperature of the mixture.

v 2. The method of claim 1 in which the liquid solvent substance is selected from the group consisting of ortho-nitro-toluene, acetophenone, ortho-chlor-aniline, technical cresol, eugenol, and nitrobenzene.

ALFRED W. FRANCIS. 

